Availability to Plants

Boron in soil ranges from 20 to 200 ppm (parts per million), but most of that is unavailable to plants. The boron that is available ranges from 0.4 to 5 ppm, and mostly as the compound boric acid, B(OH)₃. Boric acid readily leaches from soil, but as pH increases, B(OH)₃ will convert to the tetrahydroxyborate ion B(OH)₄^-, which does adsorb to clay minerals in the soil. This adsorption strengthens with increasing pH, making boron less available as pH rises — overliming a soil can lead to boron deficiencies.

Boron also adsorbs to organic matter, and this is believed to be the main source of boron to plants.

Boron in Biochemistry

Boron is primarily involved in growth and reproduction. It is essential for cell division and cell wall synthesis, and has important roles in pollination, fruit and seed development. It helps move sugars and other carbohydrates within the plant, and has roles in nitrogen metabolism, protein synthesis, and hormone regulation.

Boron Deficiency Symptoms

A boron deficiency is more likely to arise in soils with a pH above 6.5, any soil high in calcium,and in acidic, sandy soils that leach readily. A deficiency is marked by reduced growth in roots and shoots. Flowers and fruiting structures can also be affected. As with a zinc deficiency, a boron deficiency results in decreased length between stem nodes on new growth. This causes the leaves to cluster close together, and form a bushy ‘rosette’ appearance.

Brittle stems and overly-thick new leaves are signs of a boron deficiency, as are short, stubby roots with few root hairs.

Cool, overcast weather can reduce boron uptake and induce a deficiency. Applying compost to soils low in boron will have the same effect, as what little boron is available will adsorb to the organic matter in the compost.

A side-effect of a boron deficiency comes from its role in sugar transportation. Boron-deprived plants are less likely to exude carbohydrates from their roots, which in turn makes them less attractive to mycorrhizal fungi. As these are beneficial fungi which exchange phosophorus and other nutrients in exchange for those sugars, a boron deficiency may lead to reduced availability of other nutrients.

Boron Toxicity Symptoms

Boron toxicity can develop on arid and semi-arid soils as these are not subject to leaching. Boron can be toxic at levels not much higher than that required for normal growth, thus care should be exercised if applying it as a fertiliser. A boron toxicity shows on older leaves, which develop necrotic (dead), ‘burnt’ edges and tips. This necrosis moves inward between the veins until the entire leaf looks scorched and falls off prematurely.

Toxicity generally can be overcome by leaching and, if on an acid soil, by application of calcium phosphate.